1. Field of the Invention
The present invention relates to a polymer electrolyte which is applicable to solid electrolytes of batteries. Moreover, it relates to a polymer lithium battery which uses the polymer electrolyte.
2. Description of the Related Art
Polymeric materials have been found in which electrolytes such as alkali metal salts are solved in high concentrations, and which exhibit high ionic conductivity as solids. They are referred to as polymer solid electrolytes or polymer electrolytes. The polymer solid electrolytes are lightweight, are full of formability, and form solid films. Accordingly, it is expected to apply the polymer solid electrolytes as novel solid electrolytes, which exhibit elasticity and flexibility, to energy fields and electronics fields.
As ionically conducting polymers having been studied so far, ionically conducting polymers have been known which have polyether, polyester, polyamine and polysulfide linear polymers as their basic structures. Since the ionically conducting polymers, which have these chemical structures, are multi-phase crystalline polymers, their ionic conductivity is likely to be affected by the phase changes so that they exhibit low ionic conductivity at room temperature. In order to solve the problem, a variety of proposals have been made as follows:                a) polymers including ether segments which exhibit relatively high ionic conductivity;        b) polymers cross-linked with polyethers;        c) polymer electrolyte-type ion conductors; and        d) plastisized polymers.        
However, when the polymers set forth in proposal a) above have low molecular weights, they are turned into liquid state. Even when the polymers have high molecular weights, they suffer from a problem in that their mechanical strength is insufficient. Although the polymers set forth in proposal b) exhibit high mechanical strength, their formability is so low that it is difficult to form them. In the polymer electrolyte-type ion conductors set forth in proposal c), cations are captured by the paired anion sites so strongly that they exhibit low ionic conductivity. In the plastisized polymers set forth in proposal d), since organic solvents are used, they suffer from safety problems.
In order to solve the problems, a variety of polymers have reported recently in the following patent publications, for example:                Japanese Unexamined Patent Publication (KOKAI) No. 63-193,954;        Japanese Unexamined Patent Publication (KOKAI) No. 2-207,454;        Japanese Unexamined Patent Publication (KOKAI) No. 5-314,995;        Japanese Unexamined Patent Publication (KOKAI) No. 10-275,521;        Japanese Unexamined Patent Publication (KOKAI) No. 11-54,151;        Japanese Unexamined Patent Publication (KOKAI) No. 2001-55,441.        
Japanese Unexamined Patent Publication (KOKAI) No. 63-193,954 discloses an organic polymer. The organic polymer comprises a polyethylene oxide main chain, and branched polyethylene oxide in which side chains, made of ethylene oxide adducts, are introduced into the main chain byway of an ester bond. In addition, the organic polymer disclosed herein forms a composite with lithium ions, and makes a lithium ion conductive polymer electrolyte. Moreover, Japanese Unexamined Patent Publication (KOKAI) No. 2-207,454 discloses an all-solid lithium secondary battery. In the all-solid lithium secondary battery, an organic polymer is used in which side chains, made of oligoethylene oxide, are introduced into a main chain, made of polyphosphazene.
In the organic polymers, oligoethylene oxide is the side chains, and the side chains are bonded to the main chains. Accordingly, in ion conducting polymer solid electrolytes in which alkali metal salts are solved, it is difficult to maintain the mechanical properties unless the repeating units constituting the main chains exhibit sufficient mechanical strength. Moreover, in both of the organic polymers, since the main chains are made of soft polymers, they are insufficient in terms of the mechanical strength. Accordingly, they suffer from a problem regarding the formability.
Moreover, as means for enhancing the ionic conductivity of a polymer electrolyte, Japanese Unexamined Patent Publication (KOKAI) No. 5-314,995 discloses an electrolyte composite in which an insulative powder is dispersed in an amount of from 5 to 60% by volume in a solid polymer electrolyte in which Li+ is a main charge carrier. In the patent publication, Al2O3, SiO2, Fe2O3, ZrO2, CeO2, BaTiO3, PbTiO3 and Pb(Zr, Ti)O3 are named as the insulative powder, and polyethylene oxide and polypropylene oxide are used as the solid polymer electrolyte.
In addition, Japanese Unexamined Patent Publication (KOKAI) No. 10-275,521 discloses a polymer electrolyte and a process for producing the same. The polymer electrolyte is produced in the following manner. A polymer solution containing a composite oxide is turned into a film. After evaporating the solvent from the film, the film is impregnated with an electrolyte liquid. After the impregnation, the resulting polymer electrolyte exhibits a swelling degree of 2.2 or more.
In order to use the solid polymer electrolytes in polymer batteries, it is necessary to enhance the conductivity of the solid polymer electrolytes. However, depending on the types of electrolyte salts used for enhancing the conductivity, the electrolyte salts react with aluminum foils, which are used for current collectors of electrodes, to corrode aluminum foils or to form films with high resistance. Consequently, there might arise cases where it is hard for the resulting batteries to operate at room temperature.
On the other hand, in Japanese Unexamined Patent Publication (KOKAI) No. 11-54,151, the applicants of the present invention disclosed to improve the ionic conductivity by adding boroxine compounds into polymer electrolytes. When complexes of ether polymers and alkali metal salts are used as polymer electrolytes, not only cations but also anions move very well usually. By adding boroxine compounds into polymer electrolytes, the boroxine rings within the boroxine compounds make anion receptacles (or anion traps), and cations are conducted through polymer matrix so that the single ionic conduction occurs in which only the cations are transferred. Moreover, since the boroxine ring do not hinder the single ionic conduction of the cations, the cationic conductivity is enhanced. As a result, it is possible to improve the ionic conductivity at room temperature, which has been considered difficult conventionally.
Moreover, Japanese Unexamined Patent Publication (KOKAI) No. 2001-55,441 discloses an ionic conductor. The ionic conductor comprises an electrolyte salt for ionic conduction, an ionically conducting molecule including an ionic conduction molecular chain for sustaining ionic conduction, and a boroxine ring bonded to the ionic conduction molecular chain and trapping anions resulting from the electrolyte salt for ionic conduction, and a structural member for dispersion and immobilization of the ionically conducting molecule and the electrolyte salt for ionic conduction.
However, it has been required recently to further upgrade the various battery characteristics of the polymer lithium batteries which use conductive polymers.